1. Field of Invention
Embodiments relate to lenses blocking selected wavelength ranges of light, in particular to protective eyewear using such lenses, and more particularly to such protective eyewear worn by medical personnel.
2. Related Art
Medical personnel are subject to eyestrain when working under bright medical lighting. In many instances, for example, very bright, white lighting is used in surgical and examination rooms. In other instances, such as during endoscopic or arthroscopic surgery, background lighting is dimmed and medical personnel view bright video monitors. Such bright lighting typically contains ultraviolet (UV) and far violet (short violet wavelength) light that may cause both eyestrain and eye damage. Optical filters (e.g., conventional sunglasses) exist that block portions of the UV spectrum. But medical personnel must clearly see in order to carry out medical procedures, and such UV-blocking filters unacceptably darken the viewed scene. Medical personnel also identify different objects such as different tissues by color, but such darkening filters change the perceived colors of viewed objects. Thus certain qualities of light reaching the eye should be preserved to assist medical personnel.
Light is characterized by wavelength, often measured in nanometers (nm). Light radiated by most light sources contains a multiplicity of wavelengths. The spectral radiance of a specific light source is the intensity at each wavelength of light radiated by the source. Spectral radiance is typically plotted with intensity (often normalized) in the ordinate and wavelength in the abscissa.
Color may be a physical description of light, serving as a proxy for wavelength. Color may also refer to human perception of light incident on the retina. Various theories of human color perception exist. Humans can generally perceive light in a range of wavelengths from around 400 nm to around 750 nm. However, the human eye is not equally responsive to all visible wavelengths. Instead, the eye is responsive to light in three overlapping bands: a blue band centered on about 450 nm, a green band centered on about 540 nm, and a red band centered on about 600 nm. When light having a mixture of wavelengths strikes the retina, the brain perceives the mixture color as an average of the light energy received from the component wavelengths that fall within each responsive band. For example, if a mixture of blue light and yellow light is incident on the retina, the mind perceives the color of the light mixture as green, and does not distinguish the individual blue and yellow colors. Color is therefore what the human mind perceives when one or more wavelengths of light is incident on the retina.
Human eye response to light intensity in the visible spectrum varies by wavelength. FIG. 1 shows a representation of photopic (bright light) visual response of a typical human eye. Curve 102 is the strength of typical photopic visual response as a function of wavelength (xcex) and is asymmetric. Range 104 from 700-400 nm is a typical photopic visual range. Ultraviolet light is light having wavelength shorter than approximately 400 nm. As set forth in ISO 8980-3:1999(E) xe2x80x9cOphthalmic opticsxe2x80x94Uncut finished spectacle lensesxe2x80x94Part 3: Transmittance specifications and test methodsxe2x80x9d, incorporated by reference, UV-A is defined from 380 to over 315 nm and UV-B is defined from 315 to over 280 nm. Therefore, one range of UV light is approximately 400-280 nm, and is shown as UV range 106. Range 108 from 425-400 nm is a range of short violet light wavelengths (far violet) that humans can perceive, but for which the eye response is small. The intensity of light incident on the retina is perceived as brightness.
A hue is the color perceived when a single wavelength of light strikes the retina. A typical person perceives hues in groups of reds (longer than about 610 nm), oranges (about 610-590 nm), yellows (about 590-570 nm), greens (about 570-500 nm), blues (about 500-440 nm), and violets (less than about 440 nm). A single wavelength of light is perceived as having a dominant hue.
A tint identifies a particular mixture of light wavelengths and intensities (i.e., the spectral radiance distribution of a particular light). A particular tint is perceived as a corresponding color. The tint of an object is defined by how that object radiates light, or by how that object selectively reflects or absorbs wavelengths of light that strike the object. The perceived tint of an illuminated object is also determined by the intensity of light at various wavelengths coming from the object. When an object is illuminated by light of a particular tint, the illuminated object is perceived as having a color corresponding to a tint which is a combination of the tint of the light mixed with the tint of the object.
Tints are expressed in various ways. Tints may be expressed as mixtures of three primary colors (e.g., red, green, and blue). Tints may also be expressed as chroma (hue) and saturation. Saturation indicates how much or little white light is mixed with a (pure) chroma to make the tint. In the CIELAB system, tints are described by coordinates (L*, a*, b*). L* is luminosity (light or dark), a* is green-red balance, and b* is blue-yellow balance. A known spectral distribution of light can be mathematically transformed into CIELAB (L*, a*, b*) coordinates using methods known by skilled artisans.
In the CIELAB system, xcex94E is a measure of the variation between two tints, defined at coordinates (L*1, a*1, b*1) and (L*2, a*2, b*2), respectively. xcex94E is calculated by using Equation 1:
xcex94E={square root over ((xcex94L*)2+(xcex94a*)2+(xcex94b*)2)}xe2x80x83xe2x80x83[1]
where xcex94L*=L*1xe2x88x92L*2, xcex94a*=a*1xe2x88x92a*2, and xcex94b*=b*1xe2x88x92b*2. During manufacturing, for example, Equation 1 is used to compare a specified tint to a tint produced by the manufactured product (e.g., a filter). A value of 3 for such a xcex94E comparison is typically considered to be an acceptable manufacturing tolerance. For two identical tints, xcex94E is zero.
FIG. 2 is the 1931 Commission Internationale de l""Eclairage (C.I.E.) Chromaticity Diagram with typical color perceptions shown. The C.I.E. Chromaticity Diagram provides a graphical representation of tints. Since three primary colors may define a tint, if the amount of one primary color is fixed, a particular tint is defined by specifying the amounts of the other two. Consequently, two numbers are sufficient to define a mixture of three primary colors, thereby defining a tint. The defined tint is graphically shown by plotting the defining numbers (x,y) on the horizontal (x) and vertical (y) axes of the Chromaticity Diagram. For example, tint 202 is a red tint at (0.6,0.3), tint 204 is a white tint at (0.3,0.3), and tint 206 is a yellowish-green tint at (0.3,0.6). Hues (identified by wavelength in FIG. 2) are shown around the outside border of the Diagram.
A MacAdams ellipse is an area of the C.I.E. Diagram defining a boundary around a tint such that a person typically does not distinguish differences among tints inside the particular MacAdams ellipse. As shown in FIG. 2, MacAdams ellipse 208 (shown exaggerated in size) is defined around tint 204. A typical person does not distinguish tints falling within ellipse 208 from tint 204.
The color white describes light having equal intensity at all wavelengths in the visible spectrum. The color white also describes a human perception. Various color description systems define the perceived color white in various ways. For example, the Chromaticity Diagram depicts white as being in a center portion of the Diagram, as shown in FIG. 2. People identify various tints as being white, although they distinguish among various xe2x80x9cwhitesxe2x80x9d. For example, some xe2x80x9cwhitesxe2x80x9d may appear bluish; others reddish.
Objects heated to a sufficiently high temperature radiate light. A heated blackbody produces a continuous spectral radiance distribution with a single peak. As a blackbody is made hotter, the light it radiates becomes brighter (i.e., more energy is radiated at each wavelength in the radiance distribution) and the peak of the radiance distribution shifts towards shorter wavelengths.
The color temperature of a heated object, such as a lamp filament, is the temperature to which a blackbody must be heated to have approximately the spectral radiance distribution of the heated object. Thus a color temperature may be used as a model for the spectral radiance of an actual light source. Color temperature may be plotted on the Chromaticity Diagram. As shown in FIG. 3, curve 302 represents the tints of continuously increasing color temperature. A very low temperature light source radiates light perceived as being red. As the color temperature of the light source increases, the perceived color increases along line 302 from red to orange to white. The 4500xc2x0 K. color temperature is shown at tint 304. The 9300xc2x0 K. color temperature is shown at tint 306. Tint 308 represents the color temperature of an infinitely hot blackbody. Referring to FIG. 2, tints 304,306,308 are typically perceived as white, although having an increasingly bluish tint as color temperature increases.
Typical medical lamps (e.g., surgical lighting systems produced by Berchtold Corporation of Charleston, S.C.) have approximately a 4500xc2x0 K. color temperature. FIG. 4 is a graph showing spectral radiance distribution curve 402 of a 4500xc2x0 K. blackbody over the visible spectrum. The spectral radiance distribution of light sources such as cathode ray tube (CRT) displays may also be modeled using color temperature. For example, the SONY TRINITRON CRT has three selectable color temperatures of 5400xc2x0 K., 6500xc2x0 K., and 9300xc2x0 K., as well as user-defined color temperature settings. Since the CRT color temperature is higher than the surgical lamp color temperature, a particular white object displayed on the CRT will typically be perceived by an observer as having a more bluish tint than when that object is illuminated by a lower color temperature surgical lamp.
An optical filter selectively blocks light wavelengths. The filter may be described by the spectral radiance distribution of light (i.e., the tint) either blocked or passed. If the filter is described by the tint being passed, the filter""s tint is combined (added) with the tint of the light incident on the filter to determine the tint of the light exiting the filter. If the filter is described by the tint being blocked, the filter""s tint is subtracted from the tint of the light incident on the filter to determine the tint of the light exiting the filter.
Glare is stray light that interferes with a viewer""s perception of a viewed object. Glare is caused by light from objects or the environment being reflected into the eye. Several methods reduce glare.
One method of reducing glare is to view an object through a filter that tints light passing through the filter. For example, conventional sunglasses are filters that help combat glare. But such filters unacceptably darken and change the colors of viewed objects. Another method of reducing glare is to apply a layer of linear polarizing material to a lens. Light tends to be polarized parallel to the reflecting surface. Many brightly illuminated surfaces such as roads and water tend to be horizontal, and so light reflected from such surfaces has horizontal polarization. Conventional xe2x80x9cpolarizedxe2x80x9d sunglasses have a layer of polarizing material with a vertical polarization (preference) that blocks horizontally polarized light. However, such polarizing material also darkens and changes the colors of viewed objects. A third method of reducing glare is to coat a lens with a conventional anti-reflective (AR) coating. Both single layer and multi-layer AR coatings exist. AR coatings may or may not tint light passing through the coating. The amount of light reflected at an interface between air and an optical material increases when the difference in refractive index between air and the material increases. Single layer AR coatings lower the change in index of refraction between the optical surface to which it is applied and the air, thus lowering the amount of internally reflected glare light. Multi-layer AR coatings use multiple reflections and interference to cancel glare.
Lenses that include easily damaged coatings or materials can be made with a scratch-resistant outer protective layer. Such a layer protects the underlying coatings and optical substrate from damage.
Medical personnel rely on tissue color for determining medical condition and identifying physical features. They also need bright lights for a clear view while executing medical procedures. FIG. 5 illustrates a typical operating room environment. Light is depicted in FIG. 5 by arrows directed towards the surgeon""s eyes. Surgeon 502 wears protective eyewear 504. Wear of such eyewear is typically required by law or by hospital rules. Eyewear 504 may include corrective or non-corrective lenses. Light from various sources passes through lenses in eyewear 504 and is received in the surgeon""s eye. For example, bright, white light from surgical lamp 506 (e.g., 4500xc2x0 K. color temperature) is reflected into the surgeon""s eyes by patient 508. Stray light from lamp 506 also reaches the surgeon""s eyes directly, or is reflected by other objects such as cabinet 510. Light from background lighting 512 also reaches the surgeon""s eyes directly and by reflection. Video monitor (CRT) display 514 outputs an image from arthroscopic light and camera probe assembly 516 of an illuminated internal body part. Surgeon 502 may use one or more optical instruments such as a microscope (not shown) during surgery. Light passing though lenses in such instruments reaches the surgeon""s eyes.
Thus it is apparent that medical personnel working in an environment such as illustrated by FIG. 5 require protective eyewear that transmits light from many sources. Such eyewear should block harmful and straining elements (e.g., UV, far violet) of such light. At the same time, however, for procedures such as open surgery such blocking should not darken the surgeon""s view of the operating field, nor should such blocking alter object colors within the operating field. If endoscopic/arthroscopic surgery is performed, lamp 506 is turned off, background lighting 512 is dimmed, and the surgeon concentrates on display 514 which is easier to see in a darkened environment. Since the color temperature of display 514 is high (e.g., 9300xc2x0 K.), however, some objects on the display may appear to have a bluish tint. Surgeons are typically trained under 4500xc2x0 K. color temperature light, and so objects on display 514 will have a color different from that which the surgeon expects. Consequently, it is desirable to block harmful light from display 514 while at the same time preserving the perceived brightness of the display. It is also desirable to alter light output by display 514 so that colors of objects on the display are perceived to be as close as possible to colors of objects viewed with a more pure white light, such as the light produced by lamp 506. In addition, in some cases the surgeon is operating on a portion of the patient that includes several tissue types having similar tints (e.g., muscles, ligaments). If the illuminating light source and the illuminated objects both have tints shifted towards one part of the spectrum (e.g., illuminating red tissues with a low color temperature light source) the similarly tinted tissues are even more difficult to distinguish by color. It is therefore desirable to provide a way for the surgeon to more easily visually distinguish among objects having several similar tints.
A lens transmits a maximum of 2 percent of average light intensity in the ultraviolet spectrum while transmitting a minimum of 95 percent of an average light intensity in the range of wavelengths associated with brightness perception. In one embodiment, the brightness perception range is defined as a minimum photopic response (e.g., 30 percent). In some embodiments the lens also blocks light in the far violet and/or blue spectra. Blocking the ultraviolet light prevents eye damage and eyestrain. Blocking the violet and/or blue light further prevents eyestrain, especially in surgical environments in which the color blue is common. Thus a surgeon""s view of an object (e.g., an organ illuminated by a surgical spot lamp) is not unacceptably, or in some instances perceptively, darkened.
In some embodiments the lens also blocks light in the red spectrum. Such red blocking helps a person distinguish similarly red-tinted objects while viewing the objects through the lens. Some red-blocking embodiments may omit the blue-blocking feature.
A benefit of some violet- and blue-blocking embodiments is that objects illuminated by a particular color temperature light source appear when viewed through the lens to be illuminated by a lower temperature light source. When viewed through the lens, objects displayed on a relatively high color temperature video monitor are perceived to have tints closer to tints when viewed using the naked eye under a relatively lower color temperature surgical lamp. In a similar way, objects illuminated by relatively low color temperature light sources when viewed through red-blocking embodiments are perceived to be illuminated by a relatively higher color temperature light source.